1. Field of the Invention
The present invention relates to a method for forming a lens from a thermoplastic polyurethane with a specified weight percentage of urethane repeating units (—NHCOO—) having an excellent balance of optical and mechanical properties.
2. Description of the Related Art
Materials used in optical lenses must have several optical properties such as suitable refractive indexes, suitable Abbe numbers, sufficient hardness and a low yellowness index. The following survey provides the current state of the art with regard to mechanical and optical properties of some urethane polymers.
U.S. Pat. No. 5,679,756 describes the process of producing thermoplastic thiourethane-urethane copolymers (TTUC). The patent discloses that TTUC may be used as a material of optical products and may be formed by employing injection molding or compression molding techniques. The patent describes desirable material properties as a refractive index between about 1.58 and approaching 1.60 and an Abbe number above about 32. The patent also states that ophthalmic lens materials should have a shore D hardness greater than 85 and implies that a material with an Abbe number below 30 is unsuitable for use as ophthalmic lenses. Various forms of TTUC were shown in the patent to have refractive indexes between 1.579 and 1.594, Abbe numbers between 34 and 38, and Shore D Hardness numbers of 86 or 87. TTUC was compared with Dow Chemical commercial thermoplastic polyurethanes (TPUs) such as Isoplast 301 and various Pellethanes. Additionally, the patent states that although Isoplast 301 has a refractive index of 1.5952 and a shore hardness of 88, its Abbe number, 28, would make it an exceptionally poor choice for use in ophthalmic lenses. The patent also generally describes commercially available TPUs in 1995 as being unsuitable materials for ophthalmic lenses because they have low glass transition temperatures, low hardness, high yellowness, and poor refractive index and Abbe number combinations. The patent additionally discusses tinting and making the lenses of its invention photochromic.
U.S. Pat. No. 4,822,827 describes the process of producing various TPUs with high glass transition temperatures. The patent characterizes these materials as having high flexural moduli, excellent optical clarity due to their excellent light transmission extending into the UV region, and virtually no yellow coloration. The patent discloses that the optical qualities are particularly useful in medical devices, including surgical instrument trays for steam sterilization and similar devices. The patent states that the TPUs described may also be molded using injection molding and compression molding techniques among others. The patent also discloses TPUs with flexural moduli above about 1030 MPa, and specifically describes TPUs with Flexural Moduli ranging from 1741 MPa to 3478 MPa. It mentions TPUs with light transmittance between 87.17% and 90.50% and a yellow index between 1.50 and 4.13.
The prior art is directed to synthesizing various thiourethane and urethane copolymers and does not particularly address techniques for thermoforming ophthalmic lenses. Accordingly, the prior art fails to identify the correlation of NHCOO presence to acceptable mechanical properties along with lower glass transition temperatures for improved processing and cosmetics.